1. Field of the Invention
The present invention relates to an antenna used for a communication system which transmits and receives radio waves made up of specific polarization components. Specifically, the invention relates to an antenna capable of efficiently transmitting and receiving radio waves on two frequency bands in this communication system and an electronic device equipped with the antenna.
2. Description of Related Art
Recently, a variety of information including position information and road information is provided by the use of GPS (Global Positioning System) and terrestrial digital TV broadcasting via transmissions to vehicles. Also to further improve user-friendly properties and the safety, a large number of wireless communication devices have been developed and put into practical use. In terms of wireless communication with improved safety, emergency communication systems and antennas capable of transmitting and receiving vertically-polarized waves necessary for emergency communication have been developed. This type of antenna is installed on the inclined surface of automobile windshields or the like, and the direction of maximum radiation is oriented in the zenith direction. On the other hand, radio waves transmitted from a base station located at a great distance from a terminal are transmitted in a horizontal direction which is almost parallel to the ground. Therefore, it is necessary to control the antenna's maximum radiation direction so that the direction becomes horizontal to the ground.
For example, JP-A 2006-14272 discloses an example of a conventional technology, which is an antenna device capable of creating a main beam tilting from the vertical direction to the horizontal direction. This antenna is installed in a rearview mirror so that the rearview mirror functions as a reflecting plate, thereby creating a beam tilting from the vertical direction to the horizontal direction with regard to the plane. This antenna is structured such that two identical, horizontally-long slot elements are vertically disposed on one conductor, and a microstrip is connected to a portion slightly displaced from the center of the interval between two slot elements. By doing so, two slot elements are excited to occur a phase difference, and by keeping a certain distance between the antenna and the rearview mirror, the rearview mirror functions as a reflecting plate, and by synthesizing radiation from the two slots and radiation from the reflecting plate, a main beam is formed which is horizontally tilting with respect to the antenna face.
However, in this case, it can be expected that the corresponding operation can respond to one frequency band and cannot respond to two different frequency bands. To allow operation on two different frequency bands while maintaining the effects of tilting the main beam, it is necessary to provide one antenna for each frequency band. Consequently, the size of the entire antenna will, be large. Furthermore, the antenna can be installed in a rearview mirror when operating frequency is 5 GHz, however, since the wavelength is long with respect to lower operating frequency and the antenna must be large. Therefore, it becomes difficult to install the antenna in a rearview mirror, and the effects of the reflecting plate cannot be obtained, as a result, it seems that the effects of tilting the antenna face in the horizontal direction are small.
As stated above, according to the conventional technology, it is difficult to provide a small, simple antenna capable of tilting in the direction of maximum radiation on two different frequency bands.